Wire and cable insulation (which term includes primary insulation and jacketing), unless made from costly inherently flame retardant materials, is quite flammable and thus poses a hazard of fire propagation in power plants, in distribution areas, manholes, and buildings. Ignition can easily occur from overheating or arcing.
The present art of flame-retarding wire and cable insulation is accomplished in three principal ways. One approach is to utilize halogenated materials, for example chlorinated polymers such as chlorosulfonated polyethylene, neoprene, polyvinyl chloride, or the like, or halogen-containing additives such as decabromodiphenyl oxide, Dechlorane Plus.RTM., tetrabromophthalimides, chlorowaxes, or the like. Frequently such halogenated polymers or additives are boosted or "synergized" in their flame retardant activity by the further addition of antimony oxide. All of these approaches involving halogens have faced the inherent shortcoming that the evolved gases (i.e. hydrogen chloride or hydrogen bromide) in the event of fire or even merely overheating are corrosive gases, and also highly irritating to the eyes and respiratory system. A further shortcoming, more recently a basis of concern regarding halogenated flame retardants, is that they may pose an environmental hazard, being in general persistent and moreover being capable of combustion or pyrolysis under some conditions to form toxic compounds such as polyhalodibenzodioxins or polyhalodibenzofurans. Whether or not actually a hazard, at least these substances are a cause of concern and regulatory attention.
A second approach to flame retardancy of wire and cable insulation is to utilize a hydrated mineral such as alumina trihydrate or magnesium hydroxide. At rather high loadings, such minerals provide an endothermic water release under heating and burning conditions which effects a flame retardant action. The defect with these systems is the high loadings required, which render the insulation undesirably stiff and poor on abrasion. Moreover, the use of metal hydroxides as flame retardants lowers the electrical properties of the insulation.
Processing difficulties in manufacturing such highly loaded polymer systems are also experienced. Some improvement can be achieved with coupling agents such as silanes and silicones, but at substantial cost.
A third approach is to utilize phosphorus compounds plus char-forming and often intumescent additives. Typical formulations use ammonium polyphosphate as the char-forming catalysts and a resin such as dipentaerythritol, ethyleneurea-formaldehyde resin, or triazine-piperazine resin as the source of the char. Such systems have cost problems and severe electrical shortcomings. The ammonium polyphosphate component is hydrolyzable and forms electrically conductive water-soluble ammonium phosphate; moreover, most of the known char-forming resins are excessively hydrophilic and thus water-sensitive if not outright soluble.
Accordingly, there is a need for flame retardant systems for normally flammable insulation or cable jacket which does not cause corrosive or toxic emissions, which does not unduly stiffen or otherwise adversely effect the physical properties of the insulation or jacket, which permits retention of good electrical properties suitable for insulation or for jacket, and which is resistant to water, including hot water.